Understanding > Fundamental concepts > Observations VI
Collecting the electromagnetic radiation from celestial objects requires the availability of a complete optical system composed of the light collector (the telescope) and a radiation detector positioned at the telescope focus. We will consider here the optical electromagnetic radiation that is to say that the radiation may be collected by optical instruments such as telescope and which ensures a good accuracy of positioning of the source. The detectors are often specialized in a specific range of wavelengths. Pages on spectroscopy will be consulted for an overview of the electromagnetic spectrum in general and in particular the optical spectrum.
The first receiver which was used, for obvious reasons, was the eye. Its main fault is to be personal meaning that each observer interprets what he sees.
The eye is an optical system that incorporates complete optics (the cornea and the crystalline which has a focal length of about 25 mm and 8 mm at maximum aperture stopped down in accordance with the brightness through the pupil) and a receiver (the retina). The retina consists of a sensitive surface formed by a mosaic of receptors reconstructing a two-dimensional image. In the optical axis, the cone receptors are individually connected to the brain. Their maximum sensitivity is in the yellow-green spectral band at 0.55 micron (which also corresponds to the maximum flux of the Sun). Outside of the optical axis, the photosensitive receivers are most sensitive in the blue at 0.45 micrometer. The sticks are more sensitive than cones; for that reason, in order to distinguish a weak source, we must look "next" to the source. The sticks are connected in groups to the brain, reducing the sharpness of the image. Sticks and cones transform light energy into electrical short pulses whose frequency increases with illumination. This mechanism involves the vitamin A.
The quantum yield of the eye is good: the eye reacts to the arrival of four to eight photons. Unfortunately, it sums the arriving photons during about one third of a second. So, with less than 20 photons per second, the eye sees nothing.
With the invention of photography, Astronomy has a receptor which is independent of the observer and that preserves the observation. The photographic plate, unlike the eye, allows to make long exposures, storing energy in the silver grains for a very long time, which gives access to the observation of much fainter objects. It is not uncommon to have exposures of several hours. This is impossible for the eye that works instantly. This advantage is particularly critical in spectroscopy because of the dispersion of light in a large number of wavelengths, and it has, at each point of the plate, much less light. Finally, we can return the observation as often as desired.
A photographic plate is a glass plate which was plated on a layer of several tens of microns of a mixture of gelatin and silver bromide. A photon is capable of causing in this substance (emulsion), a transformation such that it becomes reducible, that is to say, if it is treated with a reducing substance (developer), it releases silver metal as black grains. Thus, the plate becomes opaque where it was illuminated. The excess of silver bromide is disolved by the developer. A negative image is obtained.
But the plate also has its shortcoming. First, the image density is not proportional to the illuminance . The " characteristic curve" of an emulsion shows that the correlation between the density and brightness of the source is not linear, which is very inconvenient to measure the relative brightness of the two stars.
Furthermore , the photographic plate does not directly provide numerical quantities. There is therefore , in the processing chain , the necessity to transfer analogic quantities into numerical ones such as the mesure of relative positions or magnitudes on the plate. This step was performed using a device called a microphotometer. It consisted of a light source that is moved over the plate and a receiver placed below the plate, in front of the source, measuring the change of light transmitted at each moment by the plate. This receiver transforming the variations of light into electricity . A final step was the conversion of the electric current into numbers. Various techniques have been used successively , the oldest and the most basic the measure by the eye of the astronomer through a kind of microscope to look at the plate.
Other defects of the photographic plate is its inability to save far infrared and its low quantum yield (it takes 1000 photons to produce a silver grain) .
The photoelectric effect is the following: when a photon of frequency n encounters a metal, it ejects an electron energy hn . If we establish a potential difference between the metal (the cathode) and another metal plate (the anode) a photon flux gives rise to a current of electrons, thus an electrical current in an external circuit connecting the anode to the cathode. To increase the sensitivity, the property of some metal layers to release more electrons are used when only one electron is hit. One can thus obtain a current of 1000000 electrons for a single electron emitted by the cathode: this is a photomultiplier photometer .
A priori, a photoelectric effect detector does not provide an image of the sky but a quantity of light. It measures the brightness of stars (magnitude) and also measure the change in magnitude of a star over time: the variation may be due to a physical phenomenon being on the celestial body (variable stars) or a phenomenon such as eclipse or occultation (double stars or solar system bodies such as the satellites of Jupiter for example). Thus, besides the two-dimensional receivers for making images, there are receivers which are used to store a parameter that varies over time. These are the photometers. In this category are also bolometers which are kinds of luxury thermometers. They are extremely sensitive receptors observing in wavelengths for which the classical photoelectric effect does not work. The bolometers are sensitive to temperature rise caused by the light they receive from the stars.
But in order to use the photoelectric effect to obtain an image, receptors have been developed, using electrostatic lenses for focusing the electron stream.
In the electronic camera , the electronic image will be focused on a photographic plate sensitive to electrons. The advantage of such a system is its better sensitivity than classic photographic plates and also the proportionality between the electron flux received and density obtained on the plate. Its disadvantage is the requirement to work in a vacuum because of the electron flux and it is very difficult to handle such an instrumentation.
The vidicon tube is a kind of conventional television camera tube. As in the electronic camera, the image will be focused by electrostatic lenses. In the case of Vidicon camera, the image will be focused on a target which will be charged by the electron flux. It will be discharged continuously by a reading system giving rise to an electric current: the video signal. The target is scanned line by line, the pulses from the signal line and the change of image are added. Traditional television standard CCIR works giving rise to 25 frames per second. Astronomers will, themselves, ask for longer exposure time to get images of very faint stars. This effect associated with the photomultiplier image intensifier system gave good results before being supplanted by the CCD target .
All these defects have found a solution in the known CCD modern receiver ( hich means Coupled Charge Device) . What is this? A CCD, as a photographic plate is a two-dimensional receiver for imaging. The silver grains are replaced by a tiny mosaic of semiconductor receptors called pixels. Each pixel records every speck of light that enters it as electrical charge. As with the photographic plate, it is therefore possible to make long exposures. The pixels are arranged like the squares of a chess board in rows and columns . The total number of cells can be very large : the CCD one million pixels (1024 x 1024) is very common. These pixels have a very high sensitivity and can also withstand, without damage, a significant illumination. The great strength of CCD is its versatility. Indeed, is included in the CCD under the form of integrated circuits, all the electronics that can "read" the number of charges of each pixel. This reading is done line by line and the pixel values corresponding are recorded on a computer disc as a matrix providing the image. The use of observational data is immediately possible. For all these reasons , CCDs have now virtually replaced all other receivers. Note that to withstand long exposure times, the receiver must be cooled (around -100°) to eliminate noise generated by the Brownian motion of atoms in the receiver.